Summary: Thorium, the radioactive metal, present in natural
sediments produced in the form of detritus minerals such as monazite,
rutile, granitic complexes, syenitic complexes, thorite, thorianite and
progenitor of U, Th and Ac series. It is used in variety of industrial
purposes, medical applications and proposed as fissile material for
nuclear energy via production of 233U from 232Th. Anthropogenic process
including modern trends in agriculture causes environmental pollution
and also affect the biochemical cycle. However, changes in pH or
different redox conditions of rocks enable a fraction of thorium to be
released in the environment eventually. This review focuses on the
radiochemical techniques, such as alpha, gamma spectroscopy as well as
inductively coupled plasma mass spectrometry (ICP-MS) that are used for
the measurement of thorium. A survey of current research activities
intend to control the incorporation of thorium in order to minimize
internal doses has been performed.

Thorium is a natural radioactive metal discovered in 1828 by the
Swedish chemist J. J. Berzelius, who named it after Thor, the Norse God
of Thunder. Thorium occurs in minerals, predominantly in
thorium-phosphate and monazite, which contains 12% thorium oxide [1].
Earth's crust (limestone and granite) contains harsh, dangerous and
long-lived isotopes of uranium and thorium. Granite, a hard igneous rock
which consists of quartz, feldspar and mica, the most durable stone
available for exterior use and also lends itself as an excellent
flooring material due to anti-slip paving in finished texture [2].
Thorium-232, the progenitor of the (4n) decays to produce other
isotopes, which occur in thorium and uranium's decay chains and its
other important isotopes 234Th, 230Th, 231Th, 227Th and 228Th exist as
members of the (4nl+), (4n2+) and (4n3+) decay series [3].

Thorium has a variety of applications, it is a silvery white metal,
retains its lustre for several months, slowly tarnishes in air becomes
grey and finally black. Thorium oxide has a melting point of 3300degC,
the highest of all known oxides. Uranium and thorium are lithophile
elements, mostly concentrated in crystal rocks with an average Th/U
ratio of 3.5 and are enriched especially in acidic igneous rocks such as
granites, pegmatites as compared to basic and ultra basic intermediate
[4-6].

Thorium is ubiquitous, its release in environment occurs both from
natural and anthropogenic sources that elevate its level over the
background. Wind blown terrestrial dust and volcanic eruptions are two
important natural sources of thorium in the air [7-8], while mining,
milling, tin processing, phosphate rock processing, phosphate fertilizer
production, coal fired utilities and industrial boilers are the
anthropogenic sources of thorium in the atmosphere [9-12].

The presence of Th In aquatic lower level animal is significant but
the bioconcentration further decreases as the trophic level decreases
[13-14]. The fate and mobility of thorium in soil is governed by the
same principles as in water, but it remains strongly sorbed to soil and
its mobility is very slow [15]. However, leaching into groundwater is
possible in some soils with low sorption capacity and the ability to
form soluble complexes. The soil/plant transfer ratio for thorium is
less than 0.01, indicating that it is not bio-concentrate in plants from
soil [16]. The plants grown at the edge of impoundments of uranium
tailings contain elevated level of thorium and the soil/plant
concentration ratio was found to be about 3 [17].

The average thorium concentration in atmosphere is reported 0.3
ng/m3 in air samples collected from different sites [18]. Different
trials in this respect were performed and the values of different
isotopes have been cited for air, surface water and ground water
[19-20]. The maximum concentration of 232Th in several fruits,
vegetables and other foods is reported to be less than 0.01 pCi/g and
the daily intakes of 230Th and 232Th for residents has been estimated to
be 0.17 and 0.11 pCi respectively. People who consume foods grown in
high background areas, reside in homes having high thorium background
levels or near radioactive waste disposal sites are exposed to higher
than normal background levels of thorium in mBq i.e 230Th-5.860 +- 0.38,
232Th-3.38 +- 0.29 and 228Th-11.8 +- 1.21.

Annual effective doses for the worker related to uranium, thorium,
tin, phosphate mining, milling and processing industries as well as gas
mantle manufacturing units are reported which are higher than normal
background levels of thorium [12-26].

Contamination of Thorium

Air

Combustion of coal, containing radio- nuclides of the uranium and
thorium series as well as 40K form a source of radioactive contamination
in the vicinity of power stations. Coal contains 0.5-7.3 ug/g thorium
produces thorium in the fly ash that depends upon the nature of the coal
burnt and emission control devices of plant [7-8, 11, 27-31]. However,
the concentration of all natural radioactive isotopes in the stack
effluents produced by coal fired power plants are usually much lower
than the natural background concentrations of these radionuclides. Fly
ash from oil and peat fired power plants are also atmospheric sources of
thorium [11, 12, 32, 33]. The intake of 232Th and 228Th was mainly due
to the vegetables, potatoes, milk and flour [34]. Elevated levels of
220Rn, 212Bi and 216Po are present at the rare earth extraction site,
although the concentrations of thorium in air particulate samples are
not significant [10, 23, 35-37].

The Environmental Protection Agency (EPA) estimated that about 0.2
Ci of 230Th is annually emitted into the air from uranium mill
facilities coal-fired utilities, by industrial boilers, phosphate rock
processing, fertilizer production facilities, and other mineral
extraction and processing facilities. About 0.084 Ci of 234Th from
uranium fuel cycle facilities and 0.0003 Ci of 232Th from underground
uranium mines are emitted into the atmosphere annually [38]. The
background conce- ntration of thorium in various environmental samples
is given in Table-1.

Water

The acidic leaching of uranium tailing piles in certain areas is a
source of 230Th in surface water and groundwater [39, 40]. The
contamination of surface waters and benthic zone by 230Th from uranium
mining, milling operations and from radium and uranium recovery plants
has been reported [41, 42]. Other industrial processes that are expected
to be sources of thorium contamination into water are phosphorus and
phosphate fertilizer production as well as processing of some tin ores.
Since both phosphate rocks and the tailings from tin ore processing
contain mainly thorium as 230Th and 232Th, respectively, discharges of
processed or unprocessed effluents and leaching from tailing piles are
also the sources of thorium in water. Leaching from landfill sites
contains uranium and thorium results in the contamination of surface
water and groundwater with thorium [34, 43, 44].

The major industrial releases of thorium to surface waters are
effluent discharges from anthropogenic sources [39, 41, 42, 45]. The
rate of deposition in atmosphere depends on the meteorological
conditions, the particle size, density as well as chemical form of
thorium particles. Thorium particles with small aerodynamic diameters (
less than 10 micron) travel long distances from their sources of
emission. In water, thorium is present as suspended matter, suspension
and mixing control the transport of particle-sorbed in water. The
concentration of dissolved thorium in water increases due to formation
of soluble complexes with carbonate, humic materials, or other ligand
[45, 46].

Soil

Thorium occurs naturally in the earth's crust at an average
lithospheric concentration of 8-12 ug/g. The typical concentration range
of naturally occurring thorium in soil is 2-12 ug/g, with an average
value 6 ug/g [10, 12, 37, 47-49]. The processes that contaminate soil
due to industrial activities are precipitation of airborne dusts and
land disposal of uranium or thorium containing wastes. According to
Environmental Protection Agency (EPA) the primary sources of thorium at
the Superfund sites are processing and extraction of thorium, uranium
and radium from ores or ore concentrates. Disposal of incandescent
lights and lanterns containing 232Th are the additional source of
thorium at waste disposal sites [50, 51].

Table-1: showing the background concentration of various matrices.

S No###Source###Concentration###References

1###Air###0.2 Ci###27

2###Coal###0.5-7.3 ug/g###27

3###Uranium fuel cycle###0.084 Ci###38

4###Underground

###uranium mines###0.0003 Ci###38

5###Water for Th-232###.0007-0.0326 mBq L -1###52

6###Water

###for Th-230###0.0008-0.0258 mBq L -1###52

7###Water for Th-228###0.0014-1.32 mBq L -1###52

8###lithosphere###8-12 ug/g 47

9###Soil###6 ug/g (general) 47

10###Soil for Th-233###30.2-48.6 Bq kg -1###52

11###Soil for Th-230###32.5-60.5 Bq kg -1###52

12###Soil for Th-228###31.0-53.0 Bq kg -1###52

Environmental Fate

To assess the environmental fate of thorium, the isotopes of
thorium with the exception of 234Th, which has short half-life, 24.1
days, are considered. The presence of 238U and 232Th concentrations in
phosphate fertilizers are of critical importance due to the concerns
that via several pathways these radionuclides reach and potentially
affect the human. These radionuclides are introduced in the environ-
ment via phosphate fertilizers and phosphor-gypsum that contain natural
radionuclides in relatively large quantities and enter into agricultural
land during cultivation. The distribution of these isotopes depend on
the distribution of the rocks from which they originate and the process
which concentrate them [53, 54]. The chain decay of these radioactive
elements emits constantly nuclear radiations a, b and g, in the
environment.

Several studies of patients treated with thorotrast, a colloidal
suspension of thorium dioxide, for radio diagnostic purposes showed an
excess incidence of cancer, primarily tumours and leukaemia, among other
medical conditions [55, 56]. Bones are excellent candidates for the in
vivo monitoring of thorium intake since it return on the bone surface
for long time. The most important pathway is through direct inhalation
of dusts resulting in radiation doses received mainly by farmers in the
farming land [57-60].

Transport and Partitioning

Thorium release in to the atmosphere from mining, milling,
processing operations of thorium and the air blown dust from uranium
tailing piles as particulate aerosol. The aerodynamic diameters of both
230Th and 232Th in atmospheric aerosols are greater than 2.5 um. The
aerodynamic diameter of 228Th, however, is less than 1.6 pm and can
travel longer distances than both 230Th and 232Th [61]. Like other
particulate matter in the atmosphere, thorium is transported from the
atmosphere to soil and water by wet and dry deposition. The deposition
of thorium through snow, rain water, dry deposition through impaction
and gravitational settling has also been observed [62, 63]. The
atmospheric residence time of thorium depends on the aerodynamic
diameter of the particles. Those with small diameters are likely to be
transported through longer distances, e.g., high 228Th/232Th activity
ratios observed in surface air are thought to be due to transport of
small particles of 282Th through long distance [49,61].

The dry deposition velocity of 212Pb, a thoron (thoron or 220Rn
itself originating from 232Th) decay product has also been reported to
be in the range 0.03-0.6 cm/se [64,m 65]. Thorium discharge in to water
as ThO2 which form sediments due to low solubility while soluble thorium
ions hydrolyze at pH above 5 forming Th(OH)4 precipitate as hydroxy
complexes, e.g., Th(OH)22+,Th2(OH)26+, Th3(OH)57+. The concentration of
soluble thorium in water is low since its hydroxy complexes are adsorbed
by particulate matter in water, get suspended as sediment, and the
concentration of soluble thorium in water is low [66-68]. The removal
from aqueous phase is expected to be higher for finer grained particles
[69, 70]. The residence time for thorium with respect to its removal by
adsorption onto particles is shorter in near shore waters than in deeper
waters, probably because of the availability of more adsorbents
(particulate matter).

The residence time vary from 1 to 70 days and the scavenging rates
varied seasonally and are inversely related to the sediment
re-suspension rate. Therefore, the removal rate was found to be
dependent on both sediment re-suspension rate and the concentration of
iron and manganese compounds, which have good adsorption properties in
water .The transport of thorium in water is principally controlled by
the flux of particles in the water, i.e., most of the thorium is carried
in the particle-sorbed state .The sediment re- suspension and mixing
control the transport of particle-sorbed in water [70, 72, 73]. Although
the concentration of dissolved thorium is low in most of the waters, but
concentration of dissolved thorium in an alkaline lake is up to 2.21
pCi/L as compared to sea water having the concentration about O.59 x
10-5 pCi/L [46]. The dissolved thorium concentration can be increased
with the formation of soluble complexes.

The anions or ligands likely to form complexes with thorium in
natural water were Co3 2-and humic materials, although some of the
thorium- citrate complexes are stable at pH above 5 [45, 46, 74-76].

The transport of thorium from water to aquatic species has also
been studied. The bio concentration factor (concentration in dry
organism/ concentration in water) in algae is as high as 9.75x104, but
the maximum value in zooplankton (calanoids and cyclopoids) is 2x104.
Moreover, it is suggested that sinking plankton and their debris account
for the sedimentation of most of the thorium from oceanic surface waters
[14]. The highest observed thorium bio concentration factor in the whole
body of rainbow trout (Salmo gairdneri) is 465. The succeeding lower bio
concentration factor, in higher trophic animals, indicates that thorium
will not biomagnify in the aquatic environment. It was also noted that
the majority of thorium body burden in fish was in the gastrointestinal
tract [13, 15].

Chelating agents produced by certain micro- organisms such as
Pseudomonas aeruginosa present in soils enhance the dissolution of
thorium in soils [77]. The transport of atmospherically deposited
thorium from soil to plants is low. The soil to plant transfer
coefficients concentration in dry plant to concentration in dry soil
were estimated to be 10-4 to 7x10-3 and 0.6x10-4 for 232Th [16, 78]. The
root systems of grasses and weeds adsorb thorium from the soil but its
transportation to the upper parts of the plant is not very extensive.
Almost l00 folds higher concentrations of all three isotopes 228Th,
230Th, and plant above ground level [79]. Ibrahim and Whicker in 1988,
predicted that under certain conditions vegetation can accumulate 230Th,
as indicated by the plant/soil concentration ratio (dry weight) of
1.9-2.9 for mixed grasses, mixed forbs and sagebrush plants grown at the
edge of uranium tailings impoundments.

Higher concentration of 228Th as compared to 232Th from various
locations in USA the concentration of 228Th was higher than that of
232Th by a factor three to seven. Higher intake of 228Th can be
explained by the in growth of this radionuclide in plants followed by
the decay of 228Ra and or 228Ac which are taken up in addition to the
direct uptake of 228Th [17]. However, it is possible that the observed
difference in the uptake of the three isotopes by plants is due to a
difference in the chemical compounds formed by these isotopes, making
one more leachable than the other under the prevailing local conditions.

Sample Preparation and Detection Methodology

The purpose of this review is to describe the analytical methods
available for the detection and to monitor thorium in environmental
media, the methods used for this purpose are given in Table-2.
Radiometric methods, such as alpha and gamma spectrometry, neutron
activation analysis (NAA), liquid scintillation spectrometry;
spectrophotometric methods, such as inductively coupled plasma atomic
emission spectrometry (ICP-AES), atomic absorption spectrometry (AAS),
voltammetry which is electrochemical method and mass spectrometry, such
as thermal ionization mass spectrometry (TIMS) and inductively coupled
plasma mass spectrometry (ICP- MS) are all being routinely used for
thorium analysis in monitoring activities. But mostly, thorium analysis
is currently performed by alpha spectrometry, gamma spectrometry, NAA,
liquid scintillation spectrometry, ICP-MS and ICP-AES [85, 86].

Table-2: showing the well established techniques for the detection
and measurement of thorium and limit of its detection in various
environmental samples.

The urine sample plus 50% by volume of concentrated HNO3 is
transferred into a beaker and the isotopic tracers 232U and 229Th are
added. The solution is heated at about 1200C until almost dryness;
concentrated NH4OH is added in the centrifuge tube and raised the pH to
7.

After centrifugation the precipitate is dissolved in 3 ml of 8N
HNO3. When the amount of the precipitate is less than 10% of the sample
volume, 5 ml of concentrated solution of Ca(NO3)2 and 5 ml of 1.6 M
H3PO4 are added [91]. A few drops of H2O2 are added in order to obtained
a clear residue, which is transferred to a centrifuge tube with 30 ml of
0.01N HNO3.Faecal samples are transferred to a quartz capsule and dried
in oven and ashed at 4000C for 24 h. Thorium-229 and 232U are added as
internal tracers prior to wet-ashing with concentrated HNO3, H2O2 and
concentrated HF in platinum capsule. Following the ashing and
evaporation, the remaining residue is fused with a 3:1 mixture of a
Li2B4O7 and LiBO2. The cooled flux is dissolved in boiling 1N HCl and
then transferred to a centrifuge tube [92, 93].

Environmental Sample Preparation

Air Sample Preparation

The particulate matters in air are collected on filter wet ashed,
fused with LiF and H2SO4, interference eliminated by complexation and
fluore- scence developed in buffered solution with 3,4,7-
trihydroxyflavanone [94-96]. Another particulate matter of air are
collected on filter wet ashed, fused with K2S2O7. Thorium is
coprecipitate with PbSO4, dissolved in ADTP and is cleaned by
complexation. Finally thorium is extracted in aqueous oxalic acid and is
electrodeposited [97-98]. Thorium in drinking water is coprecipited from
acidified sample with Fe(OH)3, clean by selective solvent extraction,
coprecipited with Al(OH)3 and the colour is develop by arsenazo (III)
reagent. After colour development, thorium is co-precipitated with LaF3
and concentrated [99, 100].

Soil and Sediment Sample Preparation

Soil samples are dissolved in HCl, HF, HNO3 and HClO4, thorium is
coprecipited with CeF, cleaned by solvent extraction, oxalate formation
and the solution is fused with NaHSO4. In the case of sediment mixture
cleaned by anion exchange resin and electrodeposited on silver disc
[101, 102, 136].

Extraction /Separation

Extraction Chromatography

Extraction chromatography is commonly used for separation of
Thorium-238 and Uranium using commercially available UTEVA resin. In
this process thorium and uranium in controlled nitric acid (1-5 mol/dm3)
were extracted with UTEVA resin and recovered with mixture containing
0.1 mol/dm3 HNO3 and 0.05-mol/dm3 oxalic acid. For the extraction of
Thorium-238 5 mol/dm3 HNO3 solution has been reported to be suitable but
thorium fluoride formation interferes with extraction of Th-238.
Addition of Al(NO3)3 and Fe(NO3)3, which have higher solubility constant
with fluoride ion than Th, that improve extractability of Th-238 in 1
mol/dm3 HNO3 sample solution [103, 104].

Liquid-Liquid Extraction Chromatography

Separation of Th(V) is done either by liquid- liquid extraction or
solid phase extraction techniques. A large number of extrants, such as
tributyl- phosphate, trioctylphosphineoxide and organo- phosphoric acid
are employed [105]. An aliquot of a sample solution containing 1.0-65.0
g of Th(V) is transferred into a 25 ml separating funnel. The pH of the
solution is adjusted at 4.5 for Th(V) using buffer solution. The mixture
is shaken with 3 ml of 1.08x10-4 M C4RAHA and the combined extracts
along with washings are diluted with ethyl acetate. The absorb- ance of
the organic phase is measured against the reagent blank at 341 nm. The
total concentration of thorium [Tho2+] species in aqueous phase
[Tho2+]aq is measured by Inductively Coupled Plasma Atomic Emission
spectrometry (ICP-AES).

The concen- tration of Th(V) extracted into organic phase,
[Tho2+]org, as a complex can be estimated by [Tho2+]org = [Tho2+](aq,
initial) -[Tho2+]aq, where [Tho2+](aq, initial) is the initial
concentration of the metal ion in the aqueous phase and the percent
extraction (%E) for Th(V) may also be calculated by [106].

Ion-Exchange Chromatography

In ion exchange chromatography thorium (IV) is eluted with 4MHCl,
the resin synthesized from XAD-4 comprises of a thioglycolate functional
group [107]. In ion exchange chromatography, number of resins has been
reported, but they have less specificity and sorption capacities due to
the poor loading of metals ions [108]. The macrocycles of the host-guest
complex are used for the complexation of several metal ions; however,
complexation with Th(V) is scanty [109]. Ligands that can selectively
bind Th(V) and strictly discriminate it from other metal ions present in
excess in monazite sand and other samples are hydroxamic acids, that
have achieved considerable importance for the separation of Th(V) [110].
The octaarmed calyx(iv)resorcinarene-hydroxamic acid, C4RAHA has been
used for the extraction and determination of Th(V) in the presence of
several interfering ions.

This extraction removes the bulk of the major elements and
precentrate Th(V) into small volume simultaneously, the extract is
directly aspirated into ICP-AES, which increases the sensitivity and
detection limit, by many fold [106].

Electrodeposition Procedure

A chemometric approach was used to evaluate and optimize the
parameters of the procedure. A balanced half- fraction factorial design
can be chosen to minimize the number of sample to be run [111]. Samples
are prepared by adding the appropriate tracer(s) to a beaker by adding 2
ml of 0.36 M NaHSO4 followed by adding concentrated HNO3 (5 ml) and
subjected to hot plate for dryness twice. Then the samples are dissolved
in 5 ml of the appropriate concentration of H2SO4 (0.50, 0.75, or 1.0
M), 4 drops of thymol blue indicator is added and finally solution is
transferred to a plating cell with a 3 ml polyethylene transfer pipette.
The planchet is rinsed with de-ionized water dried under a heating lamp
for about 5 to 10 minutes. The planchet is then counted for analyzing
alpha-spectrum [112].

The solution is transferred to a deposition cell, which contains
stain less steel disc covered with film of tri-noctylphosphinoxide/
vinol/cyclohexanone and is stirred for one hour, after which the steel
disc is transferred to a muffle furnace at 4000C for one hour. The disc
is counted in an alpha spectrometry system for 1000 min [91, 113, 114].

Radioactivity Measurement

Biological Materials

The calorimetric methods are not useful for isotope-specific
determination of thorium isotopes. Alpha spectrometric and neutron
activation analysis have potential to quantify the specific isotopes of
230Th and 232Th, respectively. Alpha spectrometry is commonly used for
the determination of 232Th and the 230Th derived from the decay of 238U
[115, 116]. In vitro monitoring methods for the analysis of thorium in
urine, feces, hair and nails have been reported that none of these
biological media is a good indicator of thorium uptake in the human. In
vivo monitoring, NaI detectors are relatively suitable for determining
thorium lungs burden. In one method, in exhaled air 220Rn is determined
as a measure of thorium lung burden. The exhaled air is passed to a
delay chamber where the positively charged decay products of thoron
(e.g. 216Po and 212Pb) are collected electrostatically with the help of
an electrode and are measured using alpha scintillation counter.

This method has prerequisite sensitivity to be used as an indicator
of thorium uptake. However, because of lack of information regarding the
thoron escape rate from the thorium particles in the lungs, the method
faces limitations accurate for indicating lung uptake of thorium [117].
Some authors have reported the levels of exhaled thoron or its decay
products in human breath [118, 119].

Environmental Samples

Standard reference materials (SRMs) for thorium in river and
freshwater lake sediment (SRM-4350B and SRM-4354), soils (SRM-4355 and
SRM-4353), coal (SRM-1632), and fly ash (SRM 1633) are available [120,
121]. Neither calorimetric nor atomic absorption/emission methods are
suitable for the determination of thorium specific isotopes, these
methods are also not sensitive enough for the quantification of trace
amounts of thorium, e.g., in seawater. The particulate phases are
filtered by inert polypropylene fiber filter and adsorption of solution
phase thorium onto MnO2-coated fiber or preconcentration of thorium on
XAD-2 resin by using adsorption of thorium-Xylenol Orange complexes
which are subjected to alpha spectrometry or neutron activation analysis
that relatively better methods for the quantification of low levels of
thorium in water [122-125].

The isotope dilution-mass spectrometric method provides the most
accurate and sensitive thorium quantification but is rarely used because
of the specialized nature and the cost of the analytical technique
[126]. The beta counting of thorium deposited on counting discs is
useful for the determination of 234Th derived from 238U [99]. The direct
gamma radiation counting with a germinium planer detector has been used
for the quantification of 228Th in grass samples [49]. The recoveries of
thorium from soil and sediment samples are usually poor and special
attention should be given to sample treatment during their analysis
[87].

Inductively coupled plasma mass spectro- metry (ICPMS) and
inductively coupled plasma optical emission spectrometry (ICP-OES) have
been routinely used for the elemental analysis of natural thorium
(232Th) in a very wide range of sample matrices for many years [126].
The techniques also offer several advantages such as shorter analysis
times and have no requirement for the separation of thorium from the
sample matrix over the more traditional radiometric methods e.g., alpha
spectro- metry, gamma spectrometry and neutron activation analysis in
the measurement of thorium [85].

In ICP-MS analysis the sample in ionized form is required, having
mass of ions, which is the characteristic of the desired analyte. The
argon plasma, where the analyte ions are formed, operates under
atmospheric pressure whereas a quadrupole mass spectrometer resolves all
of the ions formed in the plasma and operates under vacuum. The
extraction of the ions from the plasma into this vacuum system is,
therefore, a very important step in ICP-MS analysis [127]. An interface
comprising a sampler cones as well as skimmer cone are used to extract
the ions. The extracted ions are focused using a lens system, before
passing into the quadrupole mass analyzer, where they are sorted
according to their mass to charge (m/z) ratio. For the detection of
natural thorium (232Th), m/z of 232 is used and the ions are finally
detected using a channeltron electron multiplier [85].

The ICP-OES based upon the light that is emitted by the excited
atoms which is the characteristic of the element present. Atoms or ions
produced in an energised state in the plasma will spontaneously revert
to a lower energy state, with the emission photons of energy. In
quantitative measurements, the emitted energy is assumed to be
proportional to the concentration of the element present. The emitted
light passes through the entrance slit of the spectrometer and is
resolved into its components by a grating or a prism and grating
combination (echelle spectrometers). The light intensity at a specific
wavelength for each elemental line is measured either with an exit slit
and photomultiplier tube or with a semiconductor device. Thorium has
many lines in the spectrum, which can be used for quantitative analysis
and the quality of each line in the spectrum depends upon the type of
sample being analyzed.

This is because emissions from other elements present in the sample
solution can often overlap with the analyte lines. Thorium has been
measured in zirconia based ceramics, where the major matrix elements
were zirconium, cerium and titanium and found two lines appeared at
283.231 nm and 326.267 nm, respectively without interference [126, 128,
129].

Neutron Activation Analysis (NAA)

In Neutron Activation Analysis (NAA), samples are bombarded with
thermal neutrons, which render radioactive by the capture of one extra
neutron. These radioisotopes are recognized by the characteristic energy
of the gamma rays emitted as they decay with specific half-lives. The
concent- rations of particular elements are determined by measuring the
areas of the photo peaks. NAA is remarkably free of analytical problems
such as matrix effects and interferences due to photo peak overlap [130,
131]. Neutron activation analysis is relatively suitable for the
determination and measurement of thorium because it forms a radioisotope
with a long half-life and it can be determined using a late count when
the activity due to most of the other elements has been decayed.
Furthermore, the analysis is performed using large samples (typically
10-30 g), thereby reducing the risks of sample in homogeneity.

This is particularly important for thorium analysis since it is
often concentrated in minor heavy accessory minerals, which can easily
become segregated in a sample prior to analysis. The major downside of
the NAA technique is that the facility of a nuclear reactor is
compulsory and the time for the analysis of thorium is of the order of
several weeks making it unrealistic [132].

Alpha Spectrometry

This technique is able to quantify individual isotopes of thorium
independently and is based upon the measurement of energies of the
specific alpha particles emitted as a result of decay of thorium
isotopes. The essential requirement in alpha spectrometry is the
preparation of an extremely radiochemical pure source having thorium
yield from the original sample. The essential step in this analytical
technique is the sample preparation and most of the research on this
technique has been concerned with the development of protocol for
producing satisfactory sources for alpha counting [86].
Alpha-spectrometry with solid-state detectors is the benchmark of
today's methods for determining alpha emitting elements. The other
methods are being investigated that may surpass the detection
capabilities of alpha-spectrometry for long-lived radionuclides which
might be expensive and complex such as inductively coupled plasma mass
spectrometry.

Although alpha-spectrometry typically requires long counting times,
the spectra are relatively simple to analyze and the equipment is
relatively inexpensive and durable. Alpha- spectrometry affords high
confidence in the analytical results through the use of isotopic
dilution techniques such as isotopic tracers and allows analyst to
determine the quality of the chemical separation by review of the
spectra [59, 112].

Gamma Spectrometry

Thorium is frequently measured by passive gamma spectrometry, which
has an advantage over other techniques since it is a non-destructive
with low detection limits. It can also be performed directly on solid
and liquid samples. Only total thorium is determined based on the
indirect quantification of 232Th. This technique involves the
measurement of rays of specific energies emitted by the decay of progeny
nuclides 228Ac, 208Tl and 212Pb using a high purity germanium-based
detector. The disadvantages of this technique are that erroneous results
can be obtained for samples in which progeny nuclides are not in
equilibrium with 232Th and prolonged counting times, more then 20 h, may
be required for samples having low thorium concentrations [86]. The
biological and environmental samples were taken, cleaned with acetone as
well as 33% hydrogen peroxide and are dried in open air.

In the cleaning procedure the samples are cleaned once in acetone,
then in isopropanol and finally are dried in an oven at 600C for about
40 min. In order to homogenize the sample and to get a well-defined
geometry for gamma ray spectrometry, the samples are ground to a grain
size of about 1-3mm, and then weighed [133].

228Th and 228Ra contents of all the bone samples are measured by
the 208Tl (583.2 and 2614.5 keV) gamma ray lines and the 228Ac (911.2
and 969.0 keV) gamma ray lines respectively using ultra low-level gamma
ray spectrometry. For all the measurements three very sensitive
spectrometers, two coaxial HPGe detectors having 60% and 106% relative
efficiencies and one semi-planar HPGe detector of 8% relative efficiency
are used [134].

Conclusion

Sensitive analytical techniques are available for the qualitative
as well as quantitative analyses of thorium in the environmental samples
such as air, water, soil and vegetation etc. Knowledge of the different
levels of thorium compounds in environmental media can be used to
indicate human exposure to thorium through inhalation of air and
ingestion of drinking water and foods containing thorium compounds. In
drinking water and foods the concentration of thorium is very low;
however, more sensitive analytical methods are required. The
significance of ICP-MS, in precise isotope ratio measurements at
ultra-trace levels can be increase especially when multicollector and or
double focusing sector field instruments are used. This is also an
excellent tool for the determination of long- lived radionuclides in the
environment and radioactive waste.

In the environment, thorium and its compounds do not degrade or
mineralize like many organic compounds, but speciate into different
chemical compounds and form radioactive decay products. Analytical
methods for the quantification of radioactive decay products, such as
radium, radon, polonium and lead are available. The determination of
Rn-220 and its decay product in the environment may serve indirect
measure of percent compound in the environment if a secular equilibrium
is reached between thorium-232 and all its decay products.

Knowledge of the environmental transformation processes of thorium
and its compounds formation is important in understanding their
transport in environmental media. The radiation dose scenarios
described, for both workers and the population, needs to be analysed
thoroughly in order to highlight the radionuclides involved. The level
of radioactivity in the environment due to the presence of natural
radionuclides and human activities should be assessed regularly. There
is a need to develop a radioactivity level map regionally and globally.
Any nuclear activity may alter the level in specific region. Also the
detection of the radionuclides, qualitative as well as quantitative, in
the food chain is also important in order to evaluate the risk
assessment. Indeed, in recent years the non-nuclear industry has
received greater attention and is at the top of the interest of the
radiation protection community.

This paper has described the measurement and investigative
approaches, which are being pursued to determine the transport
properties of various natural and artificial products.